Define the term “specific speed” in relation to hydraulic turbines.

Short Answer:

Specific speed is a term used in hydraulic turbines to compare the performance and design of different turbines under similar conditions. It is a dimensionless number that shows the speed at which a geometrically similar turbine would run to produce one unit of power under one unit of head.

Specific speed helps in selecting the right type of turbine for a particular site condition. A low specific speed indicates suitability for high head and low flow (like Pelton), while a high specific speed suggests suitability for low head and high flow (like Kaplan).

Detailed Explanation:

Specific speed in relation to hydraulic turbines

Specific speed is a very important parameter in hydraulic turbine design and selection. It gives engineers a way to standardize and compare turbines that are different in size, shape, and operating conditions. By using specific speed, it becomes easier to predict the performance of turbines and select the most suitable type for a given site.

Definition

Specific speed (Ns) is defined as the speed of a geometrically similar turbine that would produce unit power (1 kW) under a unit head (1 meter). It combines the effects of speed, power, and head into one value and helps in turbine classification.

The formula for specific speed is:
Ns = N × √P / H^(5/4)

Where:

  • Ns = Specific speed
  • N = Rotational speed of the turbine in RPM
  • P = Power output in kilowatts
  • H = Net head in meters

Note: This formula gives specific speed in metric units and helps identify the turbine type without building a full-sized model.

Purpose and Importance

Specific speed gives insight into the internal flow behavior of the turbine. By calculating Ns, engineers can determine which turbine design will be efficient for certain head and flow conditions. It does not mean the actual speed of the turbine, but rather a comparative design figure.

  • Low specific speed (10–35): Suitable for Pelton turbines (impulse type) – for high head and low flow.
  • Medium specific speed (60–300): Suitable for Francis turbines – for medium head and flow.
  • High specific speed (300–1000+): Suitable for Kaplan turbines – for low head and high flow.

This classification helps in selecting the correct turbine quickly without trial-and-error.

Applications

  • Used during turbine selection for hydroelectric plants.
  • Helps in designing model turbines for testing and scaling up.
  • Useful in performance comparison among different turbines.
  • Aids in classification of turbines based on their flow behavior.

Factors Affecting Specific Speed

  • Rotational speed of the turbine.
  • Available head at the installation site.
  • Power requirement or turbine output.

These factors must be known accurately to compute the correct specific speed and select the matching turbine.

Engineering Use

In engineering practice, once Ns is calculated, engineers use standard turbine curves and charts to select the turbine design that matches the project site. This saves time, improves efficiency, and reduces construction and operational costs.

Conclusion:

Specific speed is a design parameter that helps in selecting and comparing hydraulic turbines. It combines speed, power, and head into a single value, guiding engineers to choose the right turbine type for any water condition. Whether it is Pelton, Francis, or Kaplan, specific speed plays a key role in achieving efficient power generation in hydroelectric systems.